Photopolymerizable compositions for solventless fiber spinning

ABSTRACT

Disclosed are methods of fiber spinning and polymer fibers that utilize multifunctional thiol and enes compounds. Also, the subject matter disclosed herein relates to uses of polymer fibers and articles prepared from such fibers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalApplication 61/472,700, filed Apr. 7, 2011, which is incorporated byreference herein in its entirety.

FIELD

The subject matter disclosed herein relates to fiber spinning andpolymer fibers. Also, the subject matter disclosed herein relates touses of polymer fibers and articles prepared from such fibers.

BACKGROUND

Fibers with micrometer and nanometer scale diameters and complexarchitectures have been of significant interest in recent years owing totheir broad scope of applications in diverse fields like regenerativemedicine, optoelectronics, sensor technology, protective clothing,filtration, catalysis, etc. Among fiber spinning techniques,electrospinning, melt blowing, and rotary jet spinning are currentlypopular for their capabilities of producing very thin fibers rangingfrom tens of nanometers to a few micrometers.

Electrospinning typically involves application of a strong electricfield (usually from about 10 to about 20 kV) to a polymer solution (upto 95% solvent by weight) that is ejected out of a syringe. At acritical voltage, when the electrostatic repulsive forces in thesolution subdue the surface tension forces, a jet of polymer solution isdriven towards a grounded collector. Rapid evaporation of solvent in theair leaves behind solid polymer fibers on the collector. As the chargedjet travels towards the collector it often experiences chaotic whippingmotion and, under some circumstances, various instabilities. Thewhipping motion is believed to amplify the stretch ratio, defined as theinitial fiber diameter divided by its final diameter, resulting in finefibers (Shin, et al., Appl Phys Lett, 78:1149-1151, 2011; Reneker, etal., J Appl Phys 87:4531-4547, 2000). Though the technique is commonlyused in research laboratories, it involves use of copious amounts ofsolvent (e.g., 80-95 wt %) and is plagued with severe environmental andeconomic challenges including solvent recycling/recovery, toxicity ofsolvents, and a lower mass throughput due to solvent evaporation (Zhou,et al., Polymer 47:7497-7505, 2006). Alternatively, one can electrospinfibers from polymer melts instead of solutions. Nevertheless, processingconstraints due to the high viscosity and low conductivity of polymermelts and the need for high temperature equipment capabilities affectsthe commercial viability.

Melt blowing involves extrusion of molten polymer through a nozzle andfurther stretching the continuous filaments with jets of hot air toyield very thin fibers often exceeding 1-2 μm in diameter. Under specialprocessing conditions, it has been recently shown that fibers below 500nm diameter could be generated from a variety of polymers using meltblowing (Ellison, et al., Polymer 48:3306-3316, 2007; Tan, et al. JNon-Newton Fluid Mech 165:892-900, 2010). Since this process does notrequire any solvent, it appears to be environmentally benign. However,it requires significant thermal energy both for melting the polymer andfor generating hot air jets with high flow rates to entrain the moltenfiber and attenuate it to finer fiber.

A spinning technique called rotary-jet or force spinning has recentlybeen developed. This technique involves spinning a polymer solution orpolymer melt through a rotating nozzle and relies on centrifugal forceto draw the fibers. This process is believed to have a much higherproduction rate of fibers as compared to that of electrospinning anddoesn't require high electric field. However the process still requiresthe polymers to be dissolved in solvents or heated to a melt prior tofiber spinning.

What is needed are methods of making fibers that are environmentallyfriendly and energy efficient. Further, methods of making fibers thatare nearly free of defects is also desirable. The subject matterdisclosed herein addresses these and other needs.

SUMMARY

In accordance with the purposes of the disclosed materials, compounds,compositions, articles, devices, and methods, as embodied and broadlydescribed herein, the disclosed subject matter, in one aspect, relatesto fiber spinning and polymer fibers. The subject matter disclosedherein also relates to uses of polymer fibers and articles prepared fromsuch fibers.

Additional advantages of the subject matter described herein will be setforth in part in the description that follows, and in part will beobvious from the description, or can be learned by practice of theaspects described below. The advantages described below will be realizedand attained by means of the elements and combinations particularlypointed out in the appended claims. It is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory only and are not restrictive.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying Figures, which are incorporated in and constitute apart of this specification, illustrate several aspects of the inventionand together with the description serve to explain the principles of theinvention.

FIG. 1 is a schematic of the electrospinning set up with UV light sourcefor in-situ photopolymerization thiol-ene compounds during fiberspinning.

FIG. 2(a) shows double bond conversion by real time FTIR spectroscopymeasurements taken in air for the photopolymerization of PETT(pentaerythritol tetrakis(3-mercaptopropionate)) and DPPA(dipentaerythritol pentaacrylate) of different thiol:ene ratios basedon: thiol:ene=1:5.6 (open circles), thiol:ene=1:4.4 (open squares),thiol:ene=1:3.4 (open triangles). FIG. 2(b) contains data from FIG. 2are-plotted to show the kinetics of curing in the first few seconds oflight exposure. Curing speed increases with increase in thiol content to40-50% double bond conversion in less than a second. Samples irradiatedat about 200 mW/cm² using 6 wt. % IRGACURE 2100™ as photoinitiator.(Data collected approximately every 0.15 sec. For clarity every 10^(th)data point has been plotted in FIG. 2a ). These plots are based oninfrared spectroscopy analysis (Shanmuganathan et al. Chemistry ofMaterials, 23: 4726-4732, 2011).

FIG. 3 is a pair of low and high magnification SEM images of thiol-enefibers made by in-situ photopolymerization of pentaerythritoltetrakis(3-mercaptopropionate) and dipentaerythritol pentaacrylate in a1:4.4 thiol to ene ratio along with 6 wt. % IRGACURE 2100™ asphotoinitiator.

FIG. 4 is a pair of low and high magnification SEM images of thiol-enefibers made by in-situ photopolymerization of pentaerythritol tetrakis(3-mercaptopropionate) and dipentaerythritol pentaacrylate in a 1:4.4thiol to ene ratio along with 6 wt. % IRGACURE 2100™ as photoinitiator.Air was blown near the collector to introduce some instability in thejet, which led to some thin fibers of less than about 10 μm.

FIG. 5(a) is a SEM image of thiol-ene fibers that were exposed to hottoluene at about 50° C. for 4 hours. The fibers remained intact andlargely unaffected demonstrating the excellent chemical stability ofthiol-ene fibers. FIG. 5(b) is a SEM image of residual thiol-ene fibersthat were leftover after heating in TGA (thermogravimetric analysis)until 1000° C. These fibers were made by in-situ photopolymerization ofpentaerythritol tetrakis (3-mercaptopropionate) and dipentaerythritolpentaacrylate in a 1:4.4 thiol to ene ratio along with 6 wt. % IRGACURE2100™ as photoinitiator.

FIG. 6 is a thermogravimetric analysis plot of a thiol-ene fiber. Thereis no significant weight loss until up to 400° C., indicatingsignificant thermal stability of the fibers.

FIG. 7 is a SEM image of thiol-ene fibers made by photopolymerization oftris[4-(vinyloxy)buytl] mellitate and pentaerythritol tetrakis(3-mercaptopropionate).

DETAILED DESCRIPTION

The materials, compounds, compositions, articles, devices, and methodsdescribed herein may be understood more readily by reference to thefollowing detailed description of specific aspects of the disclosedsubject matter and the Examples and Figures included therein.

Before the present materials, compounds, compositions, and methods aredisclosed and described, it is to be understood that the aspectsdescribed below are not limited to specific synthetic methods orspecific reagents, as such may, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular aspects only and is not intended to be limiting.

Also, throughout this specification, various publications arereferenced. The disclosures of these publications in their entiretiesare hereby incorporated by reference into this application in order tomore fully describe the state of the art to which the disclosed matterpertains. The references disclosed are also individually andspecifically incorporated by reference herein for the material containedin them that is discussed in the sentence in which the reference isrelied upon.

General Definitions

In this specification and in the claims that follow, reference will bemade to a number of terms, which shall be defined to have the followingmeanings:

Throughout the description and claims of this specification the word“comprise” and other forms of the word, such as “comprising” and“comprises,” means including but not limited to, and is not intended toexclude, for example, other additives, components, integers, or steps.

As used in the description and the appended claims, the singular forms“a,” “an,” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a thiol” includesmixtures of two or more such thiols, reference to “an ene” includesmixtures of two or more such enes, reference to “the initiator” includesmixtures of two or more such initiators, and the like.

“Optional” or “optionally” means that the subsequently described eventor circumstance can or cannot occur, and that the description includesinstances where the event or circumstance occurs and instances where itdoes not.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

Reference to parts by weight of a particular element or component in acomposition denotes the weight relationship between the element orcomponent and any other elements or components in the composition orarticle for which a part by weight is expressed. Thus, in a compoundcontaining 2 parts by weight of component X and 5 parts by weightcomponent Y, X and Y are present at a weight ratio of 2:5, and arepresent in such ratio regardless of whether additional components arecontained in the compound.

A weight percent (wt. %) of a component, unless specifically stated tothe contrary, is based on the total weight of the formulation orcomposition in which the component is included.

Certain materials, compounds, compositions, and components disclosedherein can be obtained commercially or readily synthesized usingtechniques generally known to those of skill in the art. For example,the starting materials and reagents used in preparing the disclosedmaterials are either available from commercial suppliers such as AldrichChemical Co., (Milwaukee, Wis.), Acros Organics (Morris Plains, N.J.),Fisher Scientific (Pittsburgh, Pa.), or Sigma (St. Louis, Mo.) or areprepared by methods known to those skilled in the art followingprocedures set forth in references such as Fieser and Fieser's Reagentsfor Organic Synthesis, Volumes 1-17 (John Wiley and Sons, 1991); Rodd'sChemistry of Carbon Compounds, Volumes 1-5 and Supplementals (ElsevierScience Publishers, 1989); Organic Reactions, Volumes 1-40 (John Wileyand Sons, 1991); March's Advanced Organic Chemistry, (John Wiley andSons, 4th Edition); and Larock's Comprehensive Organic Transformations(VCH Publishers Inc., 1989).

Also, disclosed herein are materials, compounds, compositions, andcomponents that can be used for, can be used in conjunction with, can beused in preparation for, or are products of the disclosed methods andcompositions. These and other materials are disclosed herein, and it isunderstood that when combinations, subsets, interactions, groups, etc.of these materials are disclosed that while specific reference of eachvarious individual and collective combinations and permutation of thesecompounds may not be explicitly disclosed, each is specificallycontemplated and described herein. For example, if a composition isdisclosed and a number of modifications that can be made to a number ofcomponents of the composition are discussed, each and every combinationand permutation that are possible are specifically contemplated unlessspecifically indicated to the contrary. Thus, if a class of thiols A, B,and C are disclosed as well as a class of sues D, E, and F and anexample of a composition A-D is disclosed, then even if each is not,individually recited, each is individually and collectivelycontemplated. Thus, in this example, each of the combinations A-E, A-F,B-D, B-E, B-F, C-D, C-E, and C-F are specifically contemplated andshould be considered disclosed from disclosure of A, B, and C; D, E, andF, and the example combination A-D. Likewise, any subset or combinationof these is also specifically contemplated and disclosed. Thus, forexample, the sub-group of A-E, B-F, and C-E are specificallycontemplated and should be considered disclosed from disclosure of A, B,and C; D, E, and F; and the example combination A-D. This conceptapplies to all aspects of this disclosure including, but not limited to,steps in methods of making and using the disclosed compositions. Thus,if there are a variety of additional steps that can be performed it isunderstood that each of these additional steps can be performed with anyspecific aspect or combination of aspects of the disclosed methods, andthat each such combination is specifically contemplated and should beconsidered disclosed.

Reference will now be made in detail to specific aspects of thedisclosed materials, compounds, compositions, articles, and methods,examples of which are illustrated in the accompanying Examples andFigures.

Disclosed herein are methods and compositions that involve the use ofthiol-ene chemistry to make fibers, e.g., thin fibers. Specifically,disclosed herein is an in-situ photopolymerization of thiol-enes withina fiber spinning set up (e.g., electrospinning setup) as an approach forgreener fiber spinning. The term thiol-ene is used herein to refer tothe disclosed photopolymerizable composition, which is used in themethods disclosed herein, since these compositions comprise thiol andene functionalized compounds. The term thiol-ene is also used herein torefer to the resulting polymer formed from polymerizing the disclosedphotopolymerizable composition.

Photopolymerizable Compositions

The photopolymerizable compositions disclosed herein comprise one ormore types of multifunctional ene compounds, one or more types ofmultifunctional thiol compounds, and one or more optionalphotoinitiators. In other aspects, the photopolymerizable compositioncomprises one or more types of multifunctional ene compounds and no or anegligible amount of thiol compounds. In still other aspect, thephotopolymerizable composition can include one or more monofunctionalene compounds or one or more monofunctional thiol compounds, or mixturesthereof, alone or in combination with one or more multifunctional onecompounds, one or more multifunctional thiol compounds, or mixturesthereof.

Other optional components disclosed herein can also be included in thephotopolymerizable compositions. The photopolymerizable compositions canbe used neat or dissolved in a suitable solvent in the methods disclosedherein; however, it is preferred that the photopolymerization be free ofsolvents. Thus in a preferred aspect, the photopolymerizablecompositions are substantially free of solvent. By “substantially free”is meant less than 5, 4, 3, 2, or 1 wt. % of the composition. Somesuitable photopolymerizable compositions are disclosed in U.S. Pat. Nos.7,521,015, 6,489,376, and 5,876,805, US Publication 2010/0064647, andInternational Publication WO95/000557, which are incorporated byreference herein for their teachings of photopolymerizable compositions.

The photopolymerizable compositions should be capable of being rapidlyphotopolymerized and cured. Generally, the disclosed photopolymerizablecompositions can be photopolymerized by irradiating the composition withUV-light (e.g., UVA at 320 to 390 nm or UVV at 395 to 445 nm), visiblelight, infrared radiation, X-rays, gamma rays, microwaves, or electronbeam radiation. The radiation can be monochromatic or polychromatic,coherent or incoherent, and sufficiently intense to generate substantialnumbers of free radicals in the photopolymerizable compositions.Suitable sources of such radiation include the sun, tungsten lamps,halogen lamps, fluorescent lamps, lasers, xenon lamps, carbon arcs,electron accelerators, cobalt 60, and mercury vapor discharge lamps.

The amount of the mono or multifunctional ene compound and the mono ormultifunctional thiol compound in the disclosed photopolymerizablecomposition may vary, but generally the molar amount of the ene compoundis in excess of the molar amount of the thiol compound. For example, themolar ratio of multifunctional ene compound to multifunctional thiolcompound can be from 10:1 to 1:1, for example, 10:1, 9:1, 8:1, 7:1, 6:1,5:1, 4:1, 3:1, 2:1, or 1:1. It is also possible that there be no ornegligible amounts or thiol compounds. Alternatively, the molar ratio ofmultifunctional ene compound to multifunctional thiol compound can befrom 1:10 to 1:1, for example, 1:10, 1:9, 1:8, 1:7, 1:6, 1:5, 1:4, 1:3,1:2, or 1:1. Alternatively, the weight amount of the multifunctional enecompound is in excess of the weight amount of the multifunctional thiolcompound. For example, the weight ratio of multifunctional ene compoundto multifunctional thiol compound can be from 10:1 to 1:10, for example,10:1, 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, 1:1, 1:10, 1:9, 1:8, 1:7,1:6, 1:5, 1:4, 1:3, or 1:2. The same ratios are also contemplated whenusing monofunctional ene or thiol compounds.

Ene

The ene compound is a component of the photopolymerizable compositionsdisclosed herein. The term “ene” is used herein as shorthand notationfor a mono or multifunctional ene compound. One or more different enecompounds can be used in the disclosed photopolymerizable compositions.A suitable multifunctional cue compound is any compound that has aplurality of pendant, internal, or terminally positioned unsaturated,free-radically polymerizable functional groups, i.e., “ene groups,” permolecule. The multifunctional ene compound can be a di-functional ene(i.e., with two ene groups per molecule), a tri-functional ene (i.e.,with three ene groups per molecule), a tetra-functional ene (i.e., withfour ene groups per molecule), a penta-functional ene (i.e., with fiveene groups per molecule), a hexa-functional ene (i.e., with six enegroups per molecule), or a higher functionalized ene compound (i.e.,with more than six ene groups per molecule). A monofunctional enecompound has a single ene group. Compounds with multiple unsaturatedcarbon-carbon or carbon-heteroatom bonds can be used, e.g., alkenes,alkynes, cyclic alkene, cyclic alkynes, allyls, carbonyls,thiocarbonyls, imines, and the like. The ene compound can be cyclic,linear, or branched.

The ene groups in the disclosed multifunctional ene compounds can beseparated from one another in a given molecule by an aliphatic group,aromatic group, ester, polyester, ether, or polyether groups.

Some suitable one compounds that can be used in the methods disclosedherein can contain vinyl ether, vinyl benzene, styrene, alkylstyrene,halostyrenes, acrylates, methactylates, acrylonitriles, vinyl chloridegroups, vinyl propionate, vinyl acetate, vinyl pivalate, vinylneononanoate; acrylamides such as N,N-dimethyl acrylamide, N,N-diethylacrylamide, N-isopropyl acrylamide, N-octyl acrylamide, and N-t-butylacrylamide, and (meth)acrylonitrile and the like. Other suitable enecompounds are norbornenes, vinyl esters, N-vinyl amides, allyl ethers,allyl triazines, allyl isocyanurates, unsaturated esters, N-substitutedmaleimides, conjugated dienes vinyl-containing ceramic precursors andother vinyl derivatives, and ‘(meth)acrylated epoxidized soybean oil.Derivatives of these compounds can also be used. These compounds are allcommercially available or synthesizable by methods known in the art. Inone example, the ene compound is a urethane (meth)acrylate. In apreferred example, the multifunctional ene compound is dipentaerythritolpentaacrylate, as shown below.

Thiol

The thiol compound is an optional component of the photopolymerizablecompositions disclosed herein. The term “thiol” is used herein asshorthand notation for a monofunctional or multifunctional thiolcompound. One or more different thiol compounds can be used in thedisclosed photopolymerizable compositions. A suitable multifunctionalthiol compound is any compound that has a plurality of pendant orterminally positioned thiol groups, i.e., SH, per molecule. Themultifunctional thiol compound can be a di-functional thiol (i.e., withtwo thiol groups per molecule), a tri-functional thiol (i.e., with threethiol groups per molecule), a tetra-functional thiol (i.e., with fourthird groups per molecule), a penta-functional thiol (i.e., with fivethiol groups per molecule), a hexa-functional thiol (i.e., with sixthiol groups per molecule), or a higher functionalized thiol compound(i.e., with more than six thiol groups per molecule). A monofunctionalthiol compound has a single thiol group. The thiol compound can becyclic, linear, or branched. Preferably, the thiol compound is free ofdisulfide linkages.

The thiol groups in the disclosed multifunctional thiol compounds can beseparated from one another in a given molecule by an aliphatic group,aromatic group, ester, polyester, ether, or polyether groups.

Some suitable thiol compounds that can be used in the methods disclosedherein are dimercaptodiethyl sulfide, 1,6-hexanedithiol,1,8-dimercapto-3,6-dithiooctane, propane-1,2,3-trithiol,1,2-bis[(2-mercaptoethyl)thio]-3-mercaptopropane,tetrakis(7-mercapto-2,5-dithiaheptyl]methane, trimethylolpropanetris(2-mercaptoacetate), pentaerythritol tetrakis(2-mercaptoacetate),ethylene glycol bis(3-mercaptopropionate), dipentaerythritolhexakis(3-mercaptopropionate), 1,4-butanediol bis(3 mercaptopropionate),tris[2-(3-mereaptopropionyloxy)ethyl] isocyanureate, tetraethyleneglycol bis(3-mercaptopropionate), ethylene glycol bisthioglycolate,trimethylolethane trithioglycolate, 1,4-butanediol bismercaptoacetate,trithiocyanuric acid and glyceryl thioglycolate, or combinations ofthese materials. Further examples of suitable thiol compounds includeαor β-mercaptocarboxylic acids such as thioglycolic acid orβ-mercaptopropionic acid. Still further examples of suitable thiolcompounds include ethylene glycol bis(thioglycolate), pentaerythritoltetrakis(3-mercaptopropionate), ethylene glycolbis(3-mercaptopropionate), trimethylolpropane tris(thiolglycolate),trimethylolpropane tris(3-mercaptopropionate), pentaerythritoltetrakis(thioglycolate), polypropylene ether glycolbis(3-mercaptopropionate), poly-2-mercaptoacetate, andpoly-3-mercaptopropionate esters (particularly, trimethylolpropanetriesters or pentaerythritol tetraesters), which are all commerciallyavailable or synthesizable by methods known in the art. In one preferredexample, the multifunctional thiol compound is pentaerythritoltetrakis(3-mercaptopropionate), as shown below.

Photoinitiator

An optional component of the photopolymerizable composition is aphotoinitiator. Suitable examples of photoinitiators includebenzophenones, acetophenone derivatives such asα-hydroxyalkylphenylketones, benzoin ethers, acylphosphonatederivatives, benzoin alkyl ethers and benzyl ketals, monoacylphosphineoxides, and bisacylphosphine oxides. Other examples of photoinitiatorsthat can be used are ethyl 2,4,6-trimethylbenzoylphenyl phosphinate, 2hydroxy-2-methyl-1-phenyl-propan-1-one,2,2-dimethoxy-2phenylacetophenone, hydroxycyclohexylphenylketone,dimethoxylphenylacetophenone, mercaptobenzothiazoles,mercaptobenzooxazoles, hydroxy ketones, phenylglyoxylates, aminoketones,metallocenes, iodonium salts and hexaryl bisimidazole, which are allcommercially available or synthesizable by methods known in the art.Additional photoinitiators are disclosed in U.S. Pat. Nos. 5,472,992 and5,218,009, which are incorporated by reference herein for theirteachings of photoinitiators. In preferred embodiment, thephotoinitiator is IRGACURE 1700™, DAROCUR 4265™, IRGACURE 819™, IRGACURE819DW™, IRGACURE 2022™ or IRGACURE 2100™ or2,2-dimethoxy-2-phenylacetophenone, which is commercially available fromCiba Additives. The photoinitiator can usually be used in an amount ofless than about 10, 9, 8, 7, 6, 5.4, 3, 2, or 1 wt. % of thephotopolymerizable composition. However, higher amounts, such as greaterthan about 10 wt. % of photoinitiator can be used.

The photoinitiator can also be part of either the ene or thiol compound.That is, the photoinitiator can be a functional group on, and thuscovalently bonded to, the ene or thiol compound.

Additional Components

The photopolymerizable compositions can comprise additional componentssuch as viscosity modifiers, surfactants, stabilizers, pigments, dyes,plasticizers, fillers, thermally stable inorganic materials,crosslinking agents, and the like. Suitable crosslinking agents can befound in U.S. Pat. No. 7,767,728, which is incorporated by referenceherein for its teachings of crosslinking agents.

In one example, a reactive diluent can be present in thephotopolymerizable compositions. A reactive diluent can be used toadjust the viscosity of the photopolymerizable composition and can be alow viscosity monomer capable of photopolymerization. Reactive diluentshave a molecular weight of less than about 550 g/mol and can be used inthe photopolymerizable composition in an amount of less than about 30,25, 20, 15, 10, 5, or 1 wt. % of the photopolymerizable composition.Suitable reactive diluents can be found in U.S. Pat. Nos. 7,521,015 and6,489,376, which are incorporated by reference herein for theirteachings of reactive diluents.

Stabilizers can also be used. Examples of suitable stabilizers arenon-acidic nitroso compounds, particularly N-nitrosohydroxylarylaminesand derivatives thereof. Alternatively, the stabilizer can be an alkenylsubstituted phenolic compound and one or more compounds selected fromthe consisting of a free radical scavenger, a hindered phenolicantioxidant and a hydroxylamine derivative. Examples of suitable alkenylsubstituted phenolic compounds include 2-propenylphenol, 4-acetoxystyrene, 2 allylphenol, isoeugenol, 2-ethoxy-5-propenylphenol,2-allyl-4-methyl-6-t-butylphenol, 2-propenyl-4-methyl-6-t-butylphenol,2-propenyl-4,6-di-t-butylphenol and 2,2′-diallyl-bisphenol A. A radicalscavenger such as p-methoxy phenol (MEHQ) and a hindered phenolicantioxidant such as butylated hydroxy toluene (BHT) can be used as well.

Inorganic compounds, sol-gel precursors and ceramic precursor monomerscould also be blended with the photopolymerizable compositions to yieldthermally stable and highly porous ceramic micro and nanofibers withother post-treatments like calcination. Examples of inorganic compoundsinclude but not limited to tetraethylorthosilicates and oxides likeAl₂O₃, TiO₂, SiO₂, PbZr₃Ti_(1-x)O₃, CuO, NiO, V₂O₅, ZnO, Co₃O₄, Nb₂O₅,MoO₃, MgTiO₃, SnO₂, BaTiO₃, ITO, GeO₂, NiFe₂O₄, and LiCoO₂ (Li, et al.,Advanced Materials, 16: 1151-1170, 2004).

Organic and inorganic micro or nanofibers, nanotubes, nanowhiskers,nanoparticles and platelets could also be added as re-inforcing agentsin the photopolymerizable composition

Fiber Spinning Methods

There have been a few attempts of photocuring electrospun fibers, butthey involved post-curing of spun fibers (Tan, et al., J Biomed MaterRes Part A, 87A:1034-1043, 2008) and employ solvents to dissolve thephotocurable polymer for processing (Gupta, et al., Macromolecules,37:9211-9218, 2004; Theron, et al., Acta Biomater 6:2434-2447, 2010).One report involves pre-heating small molecule monomers to thermallyinduce oligomer formation and later spinning fibers from this mixture(monomer acting as solvent plus oligomers) by in-situphotopolymerization (Kim, et al., Macromolecules 38:3719-3723, 2005).The main objective in these investigations was to impart chemicalstability to the final fibers by photocrosslinking.

The methods disclosed herein do not require solvents or applied heatenergy during fiber formation. Thus, pre-heating the monomers beforepolymerization, heating the fibers after polymerization, and/or heatingthe fibers before, during, or after spinning is not needed.Consequently, in certain aspects, the disclosed methods do not involvethe active application of heat energy. The disclosed methods can also bebroadly adapted to many fiber spinning techniques and used as an energyefficient and environmental friendly process to make fibrous scaffoldsfor tissue engineering and drug delivery, non-woven mats for filtrationmasks, surgical accessories and fibers for reinforced composites, amongother applications.

In the disclosed methods the photopolymerizable compositions disclosedherein, which comprise one or more types of multifunctional enecompounds, one or more types of multifunctional thiol compounds, and oneor more optional photoinitiators or additional components, isphotopolymerized while being spun into a fiber or fibers, i.e., in-situphotopolymerization. The fiber can be spun by use of an electrospinningset up, or other fiber spinning technique discussed herein or known inthe art. The photopolymerization is initiated by irradiating thephotopolymerizable composition before, during, and/or after fiberspinning. Types and sources of suitable irradiation are disclosedelsewhere herein.

Thiol-ene photopolymerizations typically follow a step-growth radicalpolymerization mechanism involving a multifunctional thiol and a widevariety of enes. However in cases where the ene monomer readilyhomopolymerizes acrylates, methacrylates, or vinyl benzenes) there is acompetition between step growth and chain growth polymerizations. Insuch cases, vinyl groups will be consumed by both homopolymerization andpropagation/chain transfer with thiols, and using a 1:1 stoichiometricmixture of thiol to ene results in less conversion of thiol compared toene. To achieve roughly equivalent conversion of functional groups, thestoichiometry should be appropriately adjusted. The chemistry andkinetics of thiol-ene polymerizations has been studied extensively byHoyle (Hoyle, et al., J Polym Sci Pol Chem 42:5301-5338, 2004) andBowman (Hoyle, et al., Angew Chem-Int Edit 49:1540-1573, 2010; Cramer,et al., J Polym Sci Pol Chem 42:5817-5826, 2004), which references areincorporated by reference herein in their entireties for their teachingsof thiol-ene polymerizations. The photoinitiator, upon activation bylight, abstracts a hydrogen atom from a thiol forming a thiyl radical.The thiyl radical propagates by attacking the ene group. This isfollowed by a chain transfer of the carbon-centered radical to anotherthiol functional group, forming a thioether linkage and regeneration ofa thiyl radical. The successive propagation/chain transfer mechanismforms the basis for the step growth polymerization. Termination occursby the coupling of any two radical species (Morgan, Polym Sci Pol Chem15:627-645, 1977, which is incorporated by reference herein in itsentirety for its teachings of thiol-ene polymerizations).

Thiol-ene photopolymerizations have a distinct advantage overtraditional acrylic photopolymerizations in that they are notsignificantly inhibited by oxygen and are generally polymerizablewithout additional photoinitiator molecules (Cramer, et al.,Macromolecules 35:5361-5365, 2002, which is incorporated by referenceherein in its entirety for its teachings of thiol-ene polymerizations).Adding a photoinitiator can still help to enhance the rate ofpolymerization.

FIG. 1 is a schematic of a setup that can be used to photopolymerizethiol-enes while electrospinning. Other fiber spinning set ups such asmelt blowing and rotary jet or force spinning can be used in thedisclosed methods. The apparatus for these various fiber formingtechniques are known and often commercially available. These set ups canbe used with the disclosed photopolymerizable compositions in a mannerconsistent with the disclosed methods. Moreover, the disclosed methodscan be performed in batches or continuously.

In certain aspects, the disclosed methods can be used with anelectrospinning set up. Both thiol-ene photopolymerizations andelectrospinning are highly capable technologies, but integrating themtogether has various challenges. One difficulty is matching the curingkinetics with the very high velocity of the fluid jet moving towardscollector. Reneker et al. reported the measured jet velocity duringfiber spinning to be about 0.5 m/s. A rough estimate of the average jetvelocity at the collector based on feed rates from the disclosedprocesses and other parameters results in about 1 m/s, which is veryclose to reported values. Combining this with the distance to thecollector, the available time for photopolymerizing the disclosedthiol-enes in-situ is on the order of 1/10th of a second. Hence, a veryhigh speed curing thiol-ene system is required in the disclosed methods.

A relevant parameter defining suitable speeds involves the gel point (α)for forming a cross-linked network of thiol-enes by light. The gel pointdepends on the functionality of the thiols and -enes and can be obtainedas follows (Hoyle, 2004),

$\begin{matrix}{\alpha = \frac{1}{\left\lbrack {{r\left( {f_{thiol} - 1} \right)}\left( {f_{ene} - 1} \right)} \right\rbrack^{1\text{/}2}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where r is the thiol to ene molar ratio, f_(thiol) is the functionalityof the thiol, and f_(ene) is the functionality of ene. For instance,when trifunctional thiols and -ene monomers are used gelation takesplace at 50% conversion of thiol and ene groups and for tetrafunctionalmonomers, gelation is at 33% conversion of functional groups. In orderto ensure in-situ photopolymerization of thiol and enes within thefraction of a second that is available in the electrospinning process, apenta- or higher functional ene and a tetra- or higher functional thiolcan be used.

In methods where the velocity of the stream is decreased, the distanceto the collector is increased, or other parameters are adjusted topermit slower conversion times, di-, tri-, and tetra-functional enes canbe used with di- and tri-functional thiols.

Another factor in photocuring is selection of the photoinitiator. Thephotoinitiator should be miscible with the thiol-ene system andfacilitate efficient high speed curing. A suitable photoinitiator isIRGACURE 2100™, a phosphine oxide based initiator whose absorption bandis strong throughout the UV and extends slightly into the visiblewavelengths. Moreover, it is a liquid initiator and mixes quickly withother thiol-ene, components making the preparation time shorter. Usingthese components, suitably rapid curing kinetics were achieved for fiberspinning as shown in the infrared spectroscopy data in FIG. 2. Othersuitable photoinitiators are disclosed herein. As such, with the use ofcertain photoinitiators, low functionalized enes and thiols can beutilized.

Another factor is that the spinning fluid should have desiredviscoelasticity to balance surface tension forces. While lower viscosityoften leads to droplets formed by surface tension driven jet breakup(electrospraying), higher viscosity can promote thicker fibers. Theviscosity range of polymer melts used in electrospinning and meltblowing are typically from about 1 to about 100 Pa-s (Tan, et al., JNon-Newton Fluid Mech 165:892-900, 2010; Dalton, et al., Polymer48:6823-6833, 2007). Viscosity can be measured with rheometers usingdynamic oscillatory sweeps or steady shear flow. Typically, the thiolenemixture is placed in a cone-plate or parallel plate geometry andviscosity measurements are made by a frequency sweep (0.01-100 Hz) inthe linear viscoelastic regime, which can be determined by dynamicstrain sweep experiments. The viscosity of the spinning mixture heredepends on the viscosity of multifunctional thiol and ene compounds andthe ratio of these components in the photopolymerizable composition.Since the thiol to ene group ratio in the photopolymerizable compositionhas an effect on curing kinetics and physical properties, a compositionthat has high curing speed and desired viscosity should be used. In athiol-acrylate system, acrylates tend to homopolymerize faster. Hence,having a 1:1 ratio of thiol to ene groups can result in incompleteconversion of thiol functional groups. Previous studies have shown thata 1:4 ratio of thiol to ene groups leads to roughly equivalentconversion of both functional groups (Cramer, et al., Macromolecules,35:5361-5165, 2002).

Generally, the viscosity of the photopolymerizable composition can befrom about 1 to about 100 Pa-s, which can be measured as explainedabove. For example, the viscosity can be from about 1 to about 90, fromabout 10 to about 80, from about 20 to about 70, from about 30 to about60, from about 40 to about 50, from about 1 to about 70, from about 1 toabout 50, from about 1 to about 30, from about 20 to about 100, fromabout 40 to about 100, from about 60 to about 100, or from about 80 toabout 100 Pa-s. A reactive diluent as disclosed herein can be used toadjust the viscosity of the photopolymerizable compositions.

Techniques for electrospinning fibers are described in a number ofpatents and the general literature. Use of commercially availableelectrospinning devices, such as those available from NanoStatics™, LLC,Circleville, Ohio, USA; and Elmarco s.r.o., Liberec, Czech Republic(e.g., using Nanospider™ technology), are preferred. A typicalelectrospinning apparatus for use in the disclosed methods includesthree primary components: a high voltage power supply, a spinneret, anda collector (effectively a grounded conductor). The spinneret is a spinelectrode that allows for extracting fibers by way of an electric field.It can be a syringe, a cylinder rotating in a melt, a capillary deviceor a conductive surface, that is connected to a feeding system forintroducing the fiber-forming self-assembling material useful in thepresent invention. A preferred system uses a pump to control the flow ofthe material out of, for example, a syringe nozzle allowing the materialto forma Taylor cone.

The disclosed photopolymerizable composition in liquid form is fed intoor onto the spinneret from, for example, the syringe at a constant andcontrolled rate using a metering pump. A high voltage (e.g., 1 to 50 kV)is applied and the drop of material at the nozzle of the syringe becomeshighly electrified. At a characteristic voltage the droplet forms aTaylor cone, and a fine jet of material develops. The fine jet is drawnto the grounded collector which is placed opposing the spinneret. Whilebeing drawn to the collector, the jet cools and hardens into fibers. Ina preferred method disclosed herein, the jet is irradiated with light inroute to the collector to in-situ photopolymerize the polymer. Thefibers are deposited on the collector as a randomly oriented, non-wovenmat or individually captured and wound-up on a roll. The fibers aresubsequently stripped from the collector.

The parameters for operating the electrospinning apparatus for effectivespinning of the disclosed photopolymerizable compositions can be readilydetermined by a person of ordinary skill in the art without undueexperimentation. By way of example, the spin electrode temperature ismaintained at about 10° C. above the melting point or temperature atwhich the photopolymerizable composition has sufficiently low viscosityto allow thin fiber formation, and the surrounding environmentaltemperature maintained at about similar temperatures. In a preferredexample, the apparatus and surrounding environment is kept at ambienttemperature and no external heat is applied to the photopolymerizablecomposition or apparatus. The applied voltage is generally about 1 toabout 120 kV, for example, from about 1 to about 50 kV. For example, theapplied electric field can be from about 10 to about 20 kV, for example,the electric field can be about 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,or 20 kV, where any of the stated values can form an upper or lowerendpoint of a range.

The electrode gap (the gap between spin electrode and collector) isgenerally between about 3 cm and about 50 cm, preferably about 3 andabout 19 cm. Preferably, the fibers can be fabricated at about ambientpressure (e.g., 1.0 atmosphere) although the pressure can be higher orlower.

The fibers prepared by the methods disclosed herein can have an averagediameter of about 1000 nm or less, more preferably about 800 nm or less,and more preferably about 600 μm or less. For example, the fiber canhave an average diameter of from about 10 μm to about 1 mm. For example,the disclosed methods can be used to make fibers from about 10 μm toabout 1 μm, from about 50 μm to about 500 μm, from about 100 μm to about300 μm, from about 500 μm to about 1 mm, from about 10 μm to about 300μm, from about 10 μm to about 100 μm, from about 300 μm to about 800 μm,or from about 500 μm to about 1 mm in average diameter. Alternatively,the polymer fiber can have an average diameter of from about 10 nm toabout 10 μm. For example, the disclosed methods can be used to makefibers from about 10 μm to about 1 nm, from about 50 nm to about 500 nm,from about 100 nm to about 300 nm, from about 500 nm to about 10 μm,from about 10 nm to about 300 μm, from about 10 nm to about 100 nm, fromabout 300 nm to about 800 nm, or from about 500 nm to about 1 μm indiameter. In other examples, the fibers can be less than about 10 μm, 1μm, 800 μm, 500 nm, or 100 nm in average diameter. In still otherexamples, the disclosed methods can be used to prepare fibers with anaverage diameter of greater than about 10 μm, 100 μm, 500 μm, 800 μm, or1 mm. Average fiber diameter for a plurality of fibers can be determinedby processing a scanning electron microscopy image thereof with, forexample, a QWin image analysis system (Leica Microsystems GmbH, 35578Wezlar, Germany).

The electrospinning techniques used herein can provide fibers with avariety of diameters and lengths. These fibers can have multiple usesand applications, such as filtration, cleaning, acoustical, medical, andenergy conservation applications, and can be used, for instance, formanufacturing medical gowns, cosmetics, sound insulation, medicalscaffolds, apparel, and barrier materials. The fibers can also besuitable for use in short-life and long-life applications such as thosedefined by INDA end-use classification (Association of Non-woven FabricsIndustry, Cary, N.C.) including, but not limited to, hygiene (diapercoverstock, adult incontinence, training pants, underpads, femininehygiene), wiping cloths, medical/surgical, filtration (air, gasses,liquids), durable paper, industrial garments, fabric softeners, homefurnishings, geotextiles, building and construction, floor coveringbackings, automotive fabrics, coatings and laminating substrates,agricultural fabrics, apparel interfacings and linings, shoes andleather, and electronic components.

EXAMPLES

The following examples are set forth below to illustrate the methods andresults according to the disclosed subject matter. These examples arenot intended to be inclusive of all aspects of the subject matterdisclosed herein, but rather to illustrate representative methods andresults. These examples are not intended to exclude equivalents andvariations of the present invention which are apparent to one skilled inthe art.

Efforts have been made to ensure accuracy with respect to numbers (e.g.,amounts temperature, etc.) but some errors and deviations should beaccounted for. Unless indicated otherwise, parts are parts by weight,temperature is in ° C. or is at ambient temperature, and pressure is ator near atmospheric. There are numerous variations and combinations ofreaction conditions, e.g., component concentrations, temperatures,pressures, and other reaction ranges and conditions that can be used tooptimize the product purity and yield obtained from the describedprocess. Only reasonable and routine experimentation will be required tooptimize such process conditions.

Example 1

Various mixtures were prepared to illustrate compositions havingsuitable viscosity and curing speed. Table 1 shows the complex viscosityof certain thiol-ene mixtures.

TABLE 1 Complex viscosity of various mixtures of pentaerythritoltetrakis (3- mercaptopropionate) (thiol) and dipentaerythritolpentaacrylate (ene). Ratio of thiol:ene Complex Viscosity* Sample groups(Pa · s) TA mixture 1 1:3.4 1.2 TA mixture 2 1:4.4 1.7 TA mixture 31:5.6 2.1 *Complex viscosity at 1 Hz and 50% strain

Example 2

The different thiol-ene mixtures shown in Table 1 along with IRGACURE2100™ (6% w/w on the total weight of thiol-ene mixture) asphotoinitiator were electrospun using a syringe needle (0.8 mm diameter)with an applied DC voltage of 18 kV, a solution feed rate of 10 mL/hrand a distance of 1.4 cm between the needle tip and grounded collector.As the thiol-ene mixture emanating from the needle was approaching thegrounded collector, it was photopolymerized with a UV-visible lightsource (350-700 nm) at an intensity of about 200 mW/cm² positionedcloser to the collector such that the light falls both on the jet nearthe collector and on the collector as well. This positioning yieldedsolid well cured fibers through efficient and timely curing of thethiol-enes. Curing the thiol-enes at a slightly earlier position beforethe collector can limit the stretching of the polymer resulting inthicker fibers, while a delayed curing can result in significantdeformation and fusion of fibers depositing on the collector. FIG. 3shows the electron microscopic images of fibers spun with a 1:4.4 ratioof thiol to ene groups (TA mixture 2). This mixture had the appropriateviscosity and curing speed to yield good solid fibers on the collectorthat were almost free of bead defects. The fiber diameter varied between15-35 μm. The fluid jet emanating from the needle was stable until ithit the collector. In typical solution electrospinning, polymer fluidejecting out of the needle is subjected to bending instabilities leadingto a whipping motion of the jet (Shin et al.; Reneker or et al.). Thisis believed to impart significant stretch on the polymer jet leading toultrathin fibers. In spinning of high viscosity polymer melts (about 5to about 100 Pa·s) by electrospinning, the bending instability issignificantly suppressed and hence relatively thicker fibers areobtained. The viscosities of the thiolene mixtures (Table 1) are in therange of melt viscosity of polymers and hence the bending instabilitiesare suppressed here as well.

Some bending instability of the jet was created by blowing air near thecollector. This created a wavy motion of fluid jet before it hit thecollector and led to some thinner fibers (less than about 10 μm). FIG. 4shows a distribution of thin and thick fibers obtained by spinning a1:4.4 mixture of thiol:ene under similar electrospinning conditions asmentioned above but with air blowing near the collector.

Example 3

The thiol-ene fibers that were obtained by in-situ photopolymerizationhad good resistance to chemical solvents when fully cured. This wasconfirmed by immersing the fibers in hot toluene at 50° C. for 4 hours.The fibers showed very good chemical resistance and their morphology wasunaffected (FIG. 5a ). The fibers also had excellent thermal stabilityshowing negligible decomposition up to 400° C. (FIG. 6).

The fibers when heated to 1000° C. in a thermogravimetric analyzerretained their morphology indicating their superior thermal stability(FIG. 5b )). These fibers were brittle. They can be made strong usefulceramic fibers by adding some other thermally stable inorganicmaterials.

Thus it has been demonstrated herein a greener fiber spinning routeusing thiol-ene photopolymerizations. Apart from being a solventlessenergy efficient process, the use of thiol-ene chemistry presents manyadvantages. The fibers can be spun in ambient atmosphere. The chemistryis versatile and tunable to particular applications. Thoughelectrospinning is illustrated in the Examples, this chemistry can bebroadly adapted in other continuous or batch spinning methods to createultrathin fibers. By modifying the chemistry and/or engineering of thefiber spinning process, fibers in the nanometer range can be obtained.Nanofibers are of significant interest in biological applications likescaffolds, tissue engineering etc and in other areas including but notlimited to filtration, gas transport and protective clothing. By blowingair in the fiber collection zone, stretching of the fiber is enhancedresulting in reduction in fiber diameter.

Other advantages which are obvious and which are inherent to theinvention will be evident to one skilled in the art. It will beunderstood that certain features and sub-combinations are of utility andmay be employed without reference to other features andsub-combinations. This is contemplated by and is within the scope of theclaims. Since many possible aspects may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth is to be interpreted as illustrative and not in alimiting sense.

1. A method for forming a polymer fiber, comprising: a. providing aphotopolymerizable composition comprising multifunctional ene compoundsand from 0 to negligible amounts of thiol compounds; b. forming a fiberfrom the photopolymerizable composition; and c. irradiating thephotopolymerizable composition with radiation during or after fiberformation to polymerize the photopolymerizable composition into apolymer, wherein the fiber consists essentially of the polymer. 2-11.(canceled)
 12. The method of claim 1, wherein the ene compounds is apenta-, hexa-, or higher functionalized ene compound.
 13. (canceled) 14.The method of claim 1, wherein the photopolymerizable compositionfurther comprises a photoinitiator.
 15. The method of claim 14, whereinthe photoinitiator is bonded to either the ene compound.
 16. (canceled)17. The method of claim 1, wherein the photopolymerizable compositionfurther comprises surfactants, pigments, dyes, plasticizers, fillers,thermally stable inorganic materials.
 18. The method of claim 1, whereinthe photopolymerizable composition is substantially free of solvent. 19.The method of claim 1, wherein the photopolymerizable composition has aviscosity of from about 1 to about 100 Pa-s.
 20. The method of claim 1,wherein the radiation is UV-light, visible light, infrared radiation,X-rays, gamma rays, microwaves, or electron beam radiation.
 21. Themethod of claim 1, wherein the fiber is formed by electrospinning. 22.The method of claim 1, wherein the fiber is formed by rotary jetspinning or by melt blowing.
 23. The method of claim 1, wherein thefiber is formed in the absence of applied heat.
 24. The method of claim1, wherein the fiber is formed at ambient atmosphere.
 25. The method ofclaim 1, wherein the fiber has an average diameter of less than about 1mm.
 26. The method of claim 1, wherein the fiber has an average diameterof from is diameters if from about 10 nm to 100 nm.
 27. (canceled) 28.The method of claim 1, wherein the ene compound is chosen from allyltriazines, allyl isocyanurates, vinyl-containing ceramic precursors,dipentaerythritol pentaacrylate, and (meth)acrylated epoxidized soybeanoil.